You can answer this by reading in Chapter 15.
If Australia is proof that people matter more than the environment, we would have to see Europeans coming to Australia and succeeding where the Aborigines failed. The Europeans would have to come and make their own society using only the things that were available to the Aborigines. This did not happen.
Instead, the Europeans only succeeded because they had plants and animals that they could bring with them. They did not domesticate Australian plants and animals. Instead, they brought their own. They did not succeed using the same things that were available to the Aborigines.
This shows (Diamond argues) that environment is more important. The Europeans succeeded because their homeland had access to many plants and animals that could be domesticated. The Aborigines failed to create a powerful and wealthy society because they lacked these resources.